Imides inclusive of polyimides have good heat resistance, resistance to chemicals, and electric insulating properties in addition to favorable mechanical characteristics so that they have been used not only as electric/electronic materials and automotive materials but also broadly as substitutes for metals and ceramics.
While polyimides are classified by skeletal chain morphology into linear polyimides (thermoplastic type) and three-dimensional network polyimides (thermosetting type), the latter thermosetting polyimides are particularly superior in heat resistance, processability, moldability, etc. and, therefore, are generally acknowledged to be of great use as electrical laminate materials, aerospace materials, and various other molding materials.
As regards the thermosetting imide resin, many versions are known, such as the resin produced by a process starting with a bismaleimide and a diamine compound, as represented by KERIMID (trademark, manufactured by Rhone-Poulenc), and the terminal double bond-containing resin such as the adduct type imide resin represented by the PMR series resin (PMR-11, trademark, manufactured by NASA).
The thermosetting imide resin has been produced typically by the method which comprises reacting a tetracarboxylic dianhydride and an acid anhydride having a carbon-carbon double bond with an organic diamine compound in an organic solvent generally at a temperature of 0.about.100.degree. C. to prepare a thermosetting amic acid solution, pouring this solution in a nonsolvent for the thermosetting amic acid, recovering the resulting precipitate, and subjecting it to cyclization for imidation.
However, when thermosetting imide resin microfine powders are to be produced from a thermosetting amic acid solution as in the above method, the resin block recovered following said imidation reaction must be pulverized by mechanical means, which adds to the complexity of the process. Moreover, a procedure by mechanical pulverization yields only coarse particles so that a monodispersed system of fine discrete particles can hardly be obtained. In addition, by the above production technology, the shape and size distribution of particles can hardly be controlled. For these reasons, there has been a persistent demand for the development of a technology for producing microfine particles of a thermosetting imide with good monodispersibility and other favorable characteristics.
An object of the present invention, therefore, is to provide thermosetting amic acid microfine particles, thermosetting imide microfine particles and crosslinked imide microfine particles each well-controlled in shape and size distribution.